An organic EL device includes a transparent anode, which has a large work function, a cathode metal, which has a small work function, and a plurality of organic thin film layers therebetween. The organic EL device is a display based on a light-emitting principle whereby, when a voltage is applied to the device in a forward direction, holes are injected from the anode to an organic layer, electrons are injected from the cathode, and recombination occurs in a light-emitting layer to thus emit light. The organic EL has high-quality panel characteristics, such as low power consumption, a wide viewing angle, a fast response speed, and a wide driving temperature range, which are required in the information age. Further, the organic EL has a merit in that costs are low compared to conventional flat panel displays, attributable to the relatively simple manufacturing process.
The purity of the organic material is a factor affecting the light emission properties of the organic EL device. When impurities are mixed with organic materials, the impurities serve as traps of carriers or cause the extinction of light to thus reduce the intensity of emitted light and the light emission efficiency. Therefore, there is a need to purify the organic materials in order to remove the impurities.
The organic material generally undergoes a purification process using a chemical method after synthesis. Examples of the chemical purification process may include recrystallization, distillation, and column chromatography. The purity of the target compound may be increased to 99% or more using the chemical purification process.
Typical examples of the purification process of the organic material include recrystallization using a solvent or recrystallization by sublimation. Recrystallization using the solvent has a merit of bulk purification of the organic material, but a drawback in that the use of the solvent means that the solvent easily penetrates organic crystals. That is, the solvent, which penetrates the organic crystals, serves as an impurity to thus reduce the light emission properties.
Other examples of the purification process include a chromatography process such as high-performance liquid chromatography (HPLC). When such a chromatography process is used to perform purification, the purity may be higher than that achieved via simple chemical purification process. However, generally, the chromatography process is only used for the purpose of analysis and is considered as a process which is unsuitable for purification of the material during mass production.
An organic light-emitting material is typically purified using a sublimation purification process. Sublimation refers to a transition phenomenon between solid and gas phases at a temperature and pressure below a triple point in a phase equilibrium diagram. Thermally decomposed by heat at normal pressure, the material is not decomposed even at a relatively high temperature under the low pressure below the triple point. The process of heating a synthesized material to separate the material from an impurity, which has a sublimation point different from that of the material, using the aforementioned characteristic without decomposing the material in a sublimation apparatus having a controllable temperature gradient is referred to as a vacuum sublimation process. The vacuum sublimation process is a purely physical process and does not rely on the use of auxiliary reagents or other chemical processes. Therefore, the vacuum sublimation process has a merit in that high-purity purification is feasible because samples are not contaminated, and accordingly, the vacuum sublimation process is known as a process that is useful for purifying an organic material for organic EL devices.
Currently, the most extensively used ultra-high-purity purification process of the organic material is a vacuum train sublimation purification process. In this process, a long tube-type chamber in a near-vacuum state is divided into a plurality of heating zones, and the materials are heated in the heating zones at different temperatures, which range from a high temperature to a low temperature, to thus ensure a temperature gradient across the materials. In the process, only the material that is deposited in a specific heating zone is used by taking advantage of the differences between the sublimation points of the materials sublimated in the chambers.
Typically, a conventional vacuum sublimation purification process is performed under the following process conditions.
(1) The heating zone is divided into three to nine zones. When the number of divided zones is small, high, intermediate, and low temperature types are used. When the number of divided zones is large, heating temperatures are set within a temperature gradient range including the zones from which the samples are taken and other zones.
(2) The sample loading zone is situated at the position opposite the vacuum pump.
(3) Although there is variation due to the properties of the materials, the initial pressure of the chamber is in the range of 10−2 to 10−6 torr before a carrier gas flows, and the pressure of the side on which the carrier gas flows is controlled to be maintained at 0.1 to several torr. Highly pure nitrogen or argon gas having no reactivity is used as the carrier gas.
(4) The loading of the sample is set so that the loading volume of the sample does not exceed ½ of the diameter of the tube if possible, in order to cause the carrier gas to move through the tube. A boat-shaped loading tool may be used.
The purpose of using the carrier gas in the conventional vacuum sublimation purification process is to improve the flowability of the sample in a vacuum sublimation state. That is, when the carrier gas is not present in a state close to a vacuum, the flowability of the sublimated sample molecules is poor, and thus solid particles are deposited on the wall of the zone, which is very close to the sample loading zone. Therefore, a basic process condition includes the use of the carrier gas in the conventional vacuum sublimation purification process.
However, the conventional vacuum sublimation purification process has some drawbacks. The biggest problem with the conventional vacuum sublimation purification process is that a predetermined zone, including an ultra-high purity material formed therein, is contaminated by the carrier gas. That is, the original samples, loaded in the sample loading zone, are scattered by the carrier gas to thus contaminate the zone including the ultra-high purity material deposited therein. Further, the carrier gas gradually transports the zone, including the ultra-high purity material deposited therein, to a third zone.
The carrier gas not only plays the aforementioned adverse role during the process, but it also strains the apparatus when the sample is loaded in a large amount. Accordingly, a portion of the sublimated sample contaminates a vacuum pump. Although a trap apparatus having a high-capacity structure is provided in order to prevent such contamination, the performance of the vacuum pump is still reduced.
Another drawback with the conventional vacuum sublimation purification process is scattering during vacuum venting. Nitrogen gas is supplied into the chamber to create normal pressure during vacuum venting. In this case, the samples, which undergo the purification process, may be scattered in the chamber. When both ends of the glass tube for purification (or quartz tube) are opened, the scattering is further aggravated, thus frequently contaminating materials that have been purified in advance.
In short, the sublimation purification process has a merit in that the raw materials are purified into the highly pure organic material using the differences between the sublimation points of the organic materials, but has various problems as follows.
(1) Since a significant amount of the organic material is exhausted to the atmosphere together with the carrier gas while sublimation and reverse sublimation are repeated during the purification process, the yield of the finally purified material to the starting material is very low and the vacuum pump is contaminated.
(2) The non-purified raw sample is scattered while the carrier gas is injected in a high vacuum, thus causing contamination. Further, the purified samples may be scattered when vacuum venting is performed in order to collect the purified organic materials after purification. Accordingly, the final purity of the target organic material is reduced.
(3) The vacuum environment of the entire system must be restored to normal pressure, after which the entire system must be stopped in order to collect the purified material after the purification process is finished. Accordingly, it is difficult to automate the entire system.
(4) Therefore, the purification process must be repeated, which increases energy consumption and thus increases the final cost of the organic material.